1. Field of the Invention
This invention relates to semiconductor manufacturing technology generally, and more specifically, to heat spreader technology for heat dissipation in a semiconductor package.
2. Description of the Related Art
There is a trend toward increasing the number of functions built into a given integrated circuit (also referred to herein as a device). This results in an increasing density of circuits in the device. Along with the increased circuit density, there is always a desire to increase the data processing rate; therefore, the clock speed of the device is increased as well. As the density of circuits and the clock speed increase, the amount of heat generated by the device increases. Unfortunately, device reliability and performance will decrease as the amount of heat that the device is exposed to increases. Therefore, it is critical that there be an efficient heat-removal system associated with integrated circuits.
FIG. 1 illustrates a typical integrated circuit and associated packaging. There are a number of methods for removing heat from integrated circuits 103, including active methods, such as fans or recirculated coolants (not shown), or passive methods, such as heat sinks 107 and heat spreaders 105. Because of the decreasing device 103 size, there is usually a need to evenly distribute heat generated by the small device 103 to the larger heat sink 107 to eliminate “hot spots” in the device. This is the function of heat spreaders 105. Heat spreaders are coupled to the integrated circuit 103 through the use of a thermally conductive material 104. These thermal interface materials 104, such as gel or grease containing metal particles to improve heat conduction, are applied in between the device 103 and the heat spreading structure 105 to improve the heat transfer from the integrated circuit 103 to the heat spreader 105. Typically, the heat spreading structure 105 will be constructed either of a ceramic material or a metal, such as aluminum or copper. Aluminum is preferred from a cost standpoint, as it is easy and cheap to manufacture; however, as the heat load that needs to be transferred increases, copper becomes the metal of choice because of its superior heat transfer characteristics (the thermal conductivity for Al is ˜250 W/m·K vs ˜395 W/M·K for Cu.) There will typically be a contiguous wall 106 around the periphery of the heat spreader, which serves as a point of attachment and support between the substrate 101 and the spreader 105. There is often a heat sink 107 attached to the heat spreader 105, to allow for the greater cooling capacity associated with the high-surface area of the heat sink 107.
With increased heat dissipation requirements, it has become necessary to improve heat spreader 105 and/or heat sink 107 performance. While improving heat sink performance through active cooling methods such as fans or recirculated liquids works well, there are a number of disadvantages associated with this solution, including bulkiness, cost and noise.
A second method for increasing heat dissipation capacity for integrated circuit packaging is through improvement of heat spreader performance. Current heat spreader materials allow for heat conduction in the range of 80-400 W/m-° K. FIG. 2 illustrates one method of increasing the rate of heat conduction in heat spreaders 201a, 201b. FIG. 2a shows a top view of a heat spreader 201a while FIG. 2b illustrates a cross-section of the same heat spreader 201b. Composites using layers of highly conductive carbon fibers 202a, 202b impregnated with carbon resin or metals 203a, 203b are known to be very effective conductors of heat. These materials also offer the added advantage of lighter weight as compared to the present materials (e.g. a density of 5.9 g/cc for Cu matrix composite versus 8.9 g/cc for copper), decreasing packaging weight, shipping cost and offering ergonomic advantages for manufacturing personnel. However, these materials have suffered from the disadvantage of being anisotropically oriented in their heat flow, thus they are typically highly conductive (>500 W/m-° K.) in only one direction. The direction of heat conduction follows the longitudinal orientation of carbon fibers, therefore the unidirectional heat flow is a result of the majority of the fibers in the composite being oriented in one direction. The aforementioned advantages are often outweighed by the disadvantage of poor heat conduction in both the second horizontal and the vertical directions.
Therefore, what is needed is an apparatus for increasing the rate of heat transfer in all three directions, allowing the rapid dissipation of heat through the heat spreader and to the heat sink.